Integrated circuits can be designed and fabricated with hundreds of semiconductor components formed on a single substrate. Each semiconductor component generally comprises conductive paths having relatively small cross-sectional areas. Because of this small dimension, a component will occasionally provide non-linear conductive properties. During different stages of manufacture of the integrated circuits, reliability tests are performed to assure that the semiconductor components will be operational within certain guidelines. One such reliability test is an “electromigration” test.
Electromigration is a phenomenon in which the metal ions of a metal conductor will tend to migrate in the direction of flow of current through the conductor. FIG. 1 illustrates an example of how metal ions 10 can migrate over time along a conductor 12, even migrating beyond the boundary of the conductor 12 itself. Factors that contribute to electromigration include the cross-sectional area of the conductor 12, current density through the conductor 12, and temperature. Over the course of time, the metal ions 10 of the conductor 12 can migrate to such a degree that voids 14 are created where the ions have left. These voids 14, when large enough, can impede the flow of current and therefore present a noticeable increase in resistance of the conductor 12. In an extreme condition, electromigration can create voids that span across the width of the conductor 12, thereby electrically isolating one portion of the conductor 12 from another and resulting in an open circuit condition.
Another problem that can result from electromigration is a situation in which some of the metal ions migrate from the conductor 12 into the surrounding semiconductor material. In this situation, if enough ions are imposed upon the surrounding semiconductor material, the material will act more like a conductor than a semiconductor and the component will not operate as it was designed.
It is therefore important to test for electromigration in order to understand the limitations of a semiconductor component or to identify ways in which the semiconductor component can be improved. Since electromigration usually becomes evident only after an extended amount of time, one way to test electromigration is by a time-accelerated method. In this method, a “test pattern” is used to represent a semiconductor component and a high current is applied to the test pattern at an elevated temperature. The test pattern is observed to determine how it changes during this relatively short amount of time so that a prediction can be made as to how the conductors of the actual semiconductor component might change during the normal life of the component. During this test, a sensor detects any changes in resistance of the test pattern as a result of the application of the high current and temperature. A noticeable change in resistance signifies a potential electromigration problem.
When electromigration tests, or other types of reliability tests, are run on a number of semiconductor components, records can be maintained of each semiconductor component defining the performance characteristics of the components under certain condition. With these records, a circuit designer can choose a component having desired characteristics for a particular design need. Usually, the records of the cells can contain information such as the maximum capacitive load of the component, frequency response, etc.
Until now, however, assumptions have been made concerning how the cells are characterized or defined. These assumptions can provide inaccurate records that falsely categorize a component as being reliable under certain conditions when in fact it may actually fail. Because of these assumptions, present-day simulation devices cannot be accurately created to accommodate all possible variables in the design of the components.
Therefore, it would be desirable to recognize some of these often-used testing assumptions and provide a technique for testing semiconductor components in order to more accurately characterize the components and overcome the inaccuracies caused by the prior assumptions. With knowledge of the inaccurate assumptions, a more thorough and more accurate testing method can be performed to provide reliable characterization analyses of semiconductor components with the assurance that the components can meet new characterization criteria as established therefor, even in a worst case scenario.